Prime Minister David Cameron announced tougher measures Monday against Britons planning to fight in Iraq and Syria, and battle-hardened jihadists who could return to launch attacks on home soil.

Cameron said his government would draw up measures to ban suspects who are British nationals from returning to the UK, while police will get enhanced powers to temporarily strip departing suspects of passports at the border.

He announced the measures in parliament after Britain raised its terror threat risk level to "severe" on Friday -- meaning an attack is thought "highly likely" -- due to fears over the situation in Iraq and Syria.

"Adhering to British values is not an option or a choice. It is a duty for all those who live in these islands so we will stand up for our values, we will in the end defeat this extremism and we will secure our way of life for generations to come," Cameron told the House of Commons.

Some 500 British jihadists are estimated to be fighting in the two countries, both of which are facing a major offensive from the Islamic State (IS) militant group.

British police made 69 arrests linked to fighting in Syria in the first half of 2014 -- a rate five times higher than in 2013 -- with offences including suspicion of travelling abroad for terrorist training.

The measures apply to suspects where there is insufficient evidence to charge or deport them and are the latest steps in years of debate since the September 11, 2001 attacks over how to handle suspected Islamic extremists in Britain.

Police will be able utilise rarely-used powers known as a Royal Prerogative to seize at Britain's borders the passports of those they suspect want to travel and fight in Syria and Iraq.

Cameron said his government would also be drawing up "a targeted discretionary power to allow us to exclude British nationals from the UK".

- Escape wearing burqa -

Fears in Britain about the number of homegrown fighters joining jihadists were fuelled last month by a graphic video showing the killing of US journalist James Foley by IS, which featured a man with a London accent.

But Cameron insisted the measures were not a "knee-jerk reaction" amid fears from his coalition partners, the Liberal Democrats, about possible civil liberties contraventions.

Civil liberties are a key part of the centre-left Liberal Democrats' political philosophy and the party was reluctant to back steps it sees as too draconian ahead of next year's general election.

Ahead of the announcement, former Liberal Democrat leader Paddy Ashdown, who was also international high representative for Bosnia and Herzegovina, warned against overreacting to the current situation.

"It is always easy to persuade frightened people to part with their liberties. It is always right for politicians who value liberty to resist attempts to increase arbitrary executive powers unless this is justified, not by magnifying fear, but by actual facts," he wrote in Sunday's Observer newspaper.

But in a blow to the Liberal Democrats, Cameron also announced a tightening of Terrorism Prevention and Investigation Measures (TPIMs).

These allow suspects who have been assessed by intelligence agencies to be tagged electronically and prevented from travelling overseas.

Cameron said the powers would now be extended to include restrictions on individual movement such as "relocation powers" and "enhanced use of exclusion zones".

There are currently no TPIMs in force, although two suspects subject to TPIMs, Ibrahim Magag and Mohammed Mohamed, absconded in 2012 and 2013 respectively.

Somali-born Mohamed, suspected of connections to Al-Qaeda linked Shebab, went missing after changing into a burqa at a mosque in west London and slipping away.